This invention relates to steam dealkylation reactions for producing benzene or its lower homologs by dealkylation of toluene and other alkylbenzenes.
A number of catalysts have been proposed for steam-dealkylating aromatic hydrocarbons, which catalysts comprise a porous carrier and at least one metal deposited on the carrier. By way of examples, there can be mentioned:
U.S.S.R. Pat. No. 213,776, wherein the catalyst comprises rhodium, nickel and alumina; PA1 U.S. Pat. No. 3,595,932, wherein the catalyst comprises a noble metal of the platinum family (platinum, palladium, rhodium, iridium, ruthenium) on a carrier consisting of alumina or combinations of alumina with nickel or cobalt; PA1 U.S. Pat. No. 3,436,433, wherein the catalyst comprises alumina, an alkali metal, ferric oxide, rhodium and chromium; PA1 U.S. Pat. No. 3,649,706 and 3,649,707, wherein the catalysts comprise mixtures of an alkali metal with ferric oxide, chromium and a metal selected from platinum, palladium and rhodium; PA1 U.S. Pat. No. 4,013,734, wherein the catalyst comprises alumina, a noble metal of the platinum family and a metal selected from vanadium, niobium and tantalum; PA1 French Pat. No. 2,317,962, wherein the catalyst comprises alumina or aluminosilicates in addition to rhodium and a metal from group IV A, particularly tin. PA1 (a) a carrier consisting essentially of alumina whose specific surface is preferably higher than 50 m.sup.2 /g, more particularly higher than 80 m.sup.2 /g, and, by weight: PA1 (b) 0.1 to 2% of at least one noble metal from group VIII of the periodic classification, selected from ruthenium, rhodium, palladium, osmium, iridium and platinum, PA1 (c) 0.05 to 2% of at least one metal from group I.sub.B selected from copper, silver and gold, copper and silver being the preferred metals, PA1 (d) 0.01 to 5% of at least one alkali metal (group I.sub.A) selected from lithium, sodium, potassium, rubidium and cesium. Potassium is the preferred metal. PA1 0.1 to 1% of rhodium PA1 0.2 to 1.5% of another noble metal from group VIII, preferably ruthenium or palladium or platinum. Rhodium amounts to about 20 to 80% by weight of all the metals of group VIII in said catalyst. PA1 0.25 to 0.65% of rhodium, PA1 0.1 to 0.9% of copper and PA1 0.5 to 3% of potassium. PA1 0.2 to 0.65% of rhodium, 0.10 to 0.90% of at least one noble metal selected from the group consisting of ruthenium, palladium and platinum, PA1 0.1 to 1.9% of copper and PA1 0.5 to 3% of potassium. PA1 Feedstock:toluene of 99.85% b.w. purity with 0.16 ppm sulfur. PA1 Pressure:7 atm. PA1 L.H.S.V.:1.5 vol. toluene per volume of catalyst per hour. PA1 H.sub.2 O/toluene:6 moles/mole PA1 Initial activity measured after 10 hours of run.
The catalysts used up to now yield fairly good results as concerns activity, but, on the one hand, their stability is insufficient and, on the other hand, their selectivity is not high enough; in addition to the conversion of the alkylaromatics to benzene, parasitic reactions of hydrocracking or steam-cracking of the aromatic ring take place, which lead to undesirable gaseous products such as CO, CO.sub.2 and CH.sub.4, and reduce both the hydrogen and aromatics yields.